init.php

<?php
/*
Script Name:  Custom Metaboxes and Fields
Contributors: WebDevStudios (@webdevstudios / webdevstudios.com)
              Justin Sternberg (@jtsternberg / dsgnwrks.pro)
              Jared Atchison (@jaredatch / jaredatchison.com)
              Bill Erickson (@billerickson / billerickson.net)
              Andrew Norcross (@norcross / andrewnorcross.com)
Description:  This will create metaboxes with custom fields that will blow your mind.
Version:      2.0.0
*/

/**
 * Released under the GPL license
 * http://www.opensource.org/licenses/gpl-license.php
 *
 * This is an add-on for WordPress
 * http://wordpress.org/
 *
 * **********************************************************************
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * **********************************************************************
 */

/************************************************************************
		You should not edit the code below or things might explode!
*************************************************************************/

/**
 * Check for newest version of CMB
 */
class cmb2_bootstrap_200 {

	/**
	 * Current version number
	 * @var   string
	 * @since 1.0.0
	 */
	const VERSION = '2.0.0';

	/**
	 * Current version hook priority
	 * Will decrement with each release
	 *
	 * @var   int
	 * @since 2.0.0
	 */
	const PRIORITY = 9999;

	public static $single = null;

	public static function go() {
		if ( null === self::$single ) { self::$single = new self(); }
		return self::$single;
	}

	private function __construct() {
		add_action( 'init', array( $this, 'include_cmb' ), self::PRIORITY );
	}

	public function include_cmb() {
		if ( ! class_exists( 'CMB2' ) ) {
			if ( ! defined( 'CMB2_VERSION' ) ) {
				define( 'CMB2_VERSION', self::VERSION );
			}
			require_once 'CMB2.php';
		}
	}

}
cmb2_bootstrap_200::go();